A turbomachine, in particular a gas turbine engine, may comprise guide vanes in order to direct gas flows generated by the compressor and turbine stages of an engine. These vanes generally act between the stages of the engine and in particular the compressor stages to direct and guide the gas flow.
The nozzle guide vane assembly is one of the most difficult areas of design because the vanes sustain the highest temperature in the engine and they must perform an efficient aerodynamic function on the hot gases which flow from the combustion chamber. The gases typically have an entry temperature between 850 and 1700° C. and may reach velocities of over 750 metres per second.
Guide vanes are often made as an annular array of separate vanes, each vane comprising an aerofoil and inner and outer platforms formed integrally with the aerofoil.
In order to maintain a high level of efficiency it is necessary to prevent leakage of the hot gases and this is of particular importance at the circumferential interfaces between the separate vanes which make up the guide vane and at the axial interfaces of the guide vane array with the preceding and following components of the turbomachine.
However, the operating conditions are such that components in the turbomachine exhibit different rates of expansion and contraction. This brings about geometric relationships that change considerably during use, which makes it difficult to seal one section of the turbomachine from another to prevent leakage of gas between the two portions.
FIG. 1 shows two components assembled together to form a portion of a circumferentially extending annular array of components. In the example of FIG. 1, the adjacent first and second components 2, 4 are guide vane elements which each comprise an aerofoil portion 6 and inner and outer platforms 8, 10 formed integrally with the aerofoil portion 6. Adjacent surfaces 12, 14 of the outer platforms 10 of the first and second components 2, 4 abut one another. It is known for the adjacent surfaces 12, 14 to be angled so as to form complementary wedges faces which provide a transitioned interface between the first and second components 2, 4. It is also known to provide a seal strip (not shown) between the adjacent surfaces 12, 14 in order to reduce the leakage through the interface between the first and second components 2, 4. Similarly, adjacent surfaces of the inner platforms 8 of the first and second components 2, 4 may be angled to provide a transitioned interface.
The adjacent surfaces 12, 14 are preferably machined at an angle which provides an approximately equal distance from the aerofoil portions of the adjacent first and second components 2, 4 to the interface between the first and second components 2, 4. In other words, each component has an inner platform 8 and outer platform 10 which is approximately equal either side of the aerofoil portion 6. This arrangement ensures that adjacent components experience equivalent radial displacement between the inner and outer platforms 8, 10. Such radial displacement being caused, primarily, by the thermal expansion of the aerofoil portion 6.
It is known to use an aerofoil portion 6 which is twisted along its length. In such an arrangement, it becomes difficult to machine the adjacent surfaces 12, 14 at an angle which provides an approximately equal distance either side of the aerofoil portion 6 for both the inner and outer platforms 8, 10. In this arrangement the adjacent surface on one side of the aerofoil portion 6 is much closer to the aerofoil portion 6 than on the other side. As a result, there is a difference in the radial displacement across the interface between the adjacent components during running conditions, which produces a scissor effect across the joint. When this occurs, the thickness of the seal strips are increased so that they act more as a structural component and hold the adjacent surfaces together. However, this puts large stresses into a relatively very thin component and the result is often that the strips are damaged and may even be torn in half.
U.S. Pat. No. 6,592,326 B2 discloses a method of connecting guide vane elements, wherein the adjacent surfaces of the outer platforms are flush and oriented radially. The outer platforms comprise flanges which are adjoined to one another using a connection means, such as a screw and nut or rivet. The radially inner and outer surfaces of the outer platform experience different amounts of axial expansion since the inner surface is exposed to hot gases and the outer surface is cooled by cooler gases. To allow for this differential in axial expansion, a gap is left between the adjacent surfaces toward the radially inner surface. This allows the expansion of the inner surface without putting unnecessary stress on the connection during operating conditions. However the method of U.S. Pat. No. 6,592,326 B2 does not deal with the differential in radial expansion experienced by guide vanes with a twisted aerofoil portion. In such an arrangement, the differential in radial expansion creates a shear stress on the connection means and may lead to failure of the connection. In addition, the process of connecting the components adds time to the manufacture of the guide vane and also in any subsequent disassembly and repair.
It is an object of the present invention to provide an improved means of preventing relative radial displacement of the outer platforms.